Packing materials for liquid chromatography (LC) are generally classified into two types: organic materials, e.g., polydivinylbenzene, and inorganic materials typified by silica. Many organic materials are chemically stable against strongly alkaline and strongly acidic mobile phases, allowing flexibility in the choice of mobile phase pH. However, organic chromatographic materials generally result in columns with low efficiency, leading to inadequate separation performance, particularly with low molecular-weight analytes. Furthermore, many organic chromatographic materials shrink and swell when the composition of the mobile phase is changed. In addition, most organic chromatographic materials do not have the mechanical strength of typical chromatographic silicas.
Due in large part to these limitations, silica is the material most widely used in High Performance Liquid Chromatography (HPLC). The most common applications employ silica that has been surface-derivatized with an organic functional group such as octadecyl (C18), octyl (C8), phenyl, amino, cyano, etc. As stationary phases for HPLC, these packing materials result in columns that have high efficiency and do not show evidence of shrinking or swelling.
Silica is characterized by the presence of silanol groups on its surface. During a typical derivatization process such as reaction with octadecyldimethylchlorosilane, at least 50% of the surface silanol groups remain unreacted. These residual silanol groups interact with basic and acidic analytes via ion exchange, hydrogen bonding and dipole/dipole mechanisms. The residual silanol groups create problems including increased retention, excessive peak tailing and irreversible adsorption of some analytes. Another drawback with silica-based columns is their limited hydrolytic stability. First, the incomplete derivatization of the silica leaves patches of bare silica surface which can be readily dissolved under alkaline conditions, generally pH greater than 8.0, leading to the subsequent collapse of the chromatographic bed. Secondly, the bonded phase can be stripped off the surface under acidic conditions, generally pH less than 2.0, and eluted off the column by the mobile phase, causing loss of analyte retention, and an increase in the concentration of surface silanol groups.
To overcome the problems of residual silanol group activity and hydrolytic instability of silica-based stationary phases, many methods have been tried including use of ultrapure silica, carbonized silica, coating of the silica surface with polymeric materials, endcapping free silanol groups with a short-chain reagent such as trimethylsilane, and the addition of suppressors such as amines to the eluant. These approaches have not proven to be completely satisfactory in practice.
One approach is disclosed in U.S. Pat. No. 4,017,528. A process for preparing a xe2x80x9chybridxe2x80x9d silica is described wherein an alkyl functionality is coupled into both the skeleton structure and the surface of the silica. According to the ""528 patent, the hybrid silica can be prepared by two methods. In the first method, a mixture of tetraethoxysilane (TEOS) and an organotriethoxysilane, e.g., alkyltriethoxysilane, is co-hydrolyzed in the presence of an acid catalyst to form a liquid material containing polyorganoethoxysiloxane (POS) oligomers, e.g., polyalkylethoxysiloxane oligomers. Then, the POS is suspended in an aqueous medium and gelled into porous particles in the presence of a base catalyst. In the second method, the material is prepared by a similar procedure except that the suspension droplet is a mixture of organotriethoxysilane, e.g., alkyltriethoxysilane, and polyethoxysiloxane (PES) oligomers; the latter is prepared by partial hydrolysis of TEOS.
There are several problems associated with the ""528 hybrid material. First, these hybrid materials contain numerous micropores, i.e., pores having a diameter below 34 xc3x85. It is known that such micropores inhibit solute mass transfer, resulting in poor peak shape and band broadening.
Second, the pore structure of the ""528 hybrid material is formed because of the presence of ethanol (a side product of the gelation process) within the suspension oil droplets. The pore volume is controlled by the molecular weight of the POS or PES. The lower the molecular weight of the POS or PES, the more ethanol is generated during the gelation reaction, and subsequently a larger pore volume is produced. However, part of the ethanol generated during the gelation is able to diffuse into the aqueous phase by partition. If the amount of the ethanol generated within the suspension droplets is too great, the partition of the ethanol will cause the structure of the droplets to collapse, forming irregularly-shaped particles as opposed to spherical particles. Therefore, the strategy to control the pore volume of the hybrid material described in the ""528 patent has certain limitations, particularly for preparing highly spherical hybrid materials with a pore volume greater than about 0.8 cm3/g. It is well known in the art that irregularly-shaped materials are generally more difficult to pack than spherical materials. It is also known that columns packed with irregularly-shaped materials generally exhibit poorer packed bed stability than spherical materials of the same size.
Thirdly, the ""528 hybrid materials are characterized by an inhomogeneous particle morphology, which contributes to undesirable chromatographic properties, including poor mass transfer properties for solute molecules. This is a consequence of the gelation mechanism, where the base catalyst reacts rapidly near the surface of the POS droplet, forming a xe2x80x9cskinnedxe2x80x9d layer having very small pores. Further gelation in the interior of the droplet is then limited by the diffusion of catalyst through this outer layer towards the droplet center, leading to particles having skeletal morphologies and hence pore geometries, e.g., xe2x80x9cshell shapedxe2x80x9d, which can vary as a function of location between the particle center and outer layer.
The present invention relates to a novel material for chromatographic separations, processes for its preparation, and separations devices containing the chromatographic material. In particular, one aspect of the invention is a porous inorganic/organic hybrid material, comprising porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry.
Another aspect of the invention is a porous inorganic/organic hybrid material, comprising porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry. The particles are surface modified with a surface modifier having the formula Za(Rxe2x80x2)bSixe2x80x94R, where Z=Cl, Br, I, C1-C5 alkoxy, dialkylamino or trifluoromethanesulfonate; a and b are each an integer from 0 to 3 provided that a+b=3; Rxe2x80x2 is a C1-C6 straight, cyclic or branched alkyl group, and R is a functionalizing group.
An additional aspect of the invention is a method of preparation of porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry, comprising the steps of
a) forming porous inorganic/organic hybrid particles,
b) modifying the pore structure of said porous particles.
In another aspect of the invention, the invention is a method of preparation of porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry, comprising the steps of
a) forming porous inorganic/organic hybrid particles,
b) modifying the pore structure of the porous particles, and
c) surface modifying the porous particles
wherein the surface modification step includes surface modifying the porous particles with a surface modifier having the formula Za(Rxe2x80x2)bSixe2x80x94R, where Z=Cl, Br, I, C1-C5 alkoxy, dialkylamino or trifluoromethanesulfonate; a and b are each an integer from 0 to 3 provided that a+b=3; Rxe2x80x2 is a C1-C6 straight, cyclic or branched alkyl group, and R is a functionalizing group.
Yet another aspect of the invention is a separations device having a stationary phase comprising porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry.
Another aspect of the invention is a separations device having a stationary phase comprising porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry, wherein the particles have been surface modified with a surface modifier having the formula Za(Rxe2x80x2)bSixe2x80x94R, where Z=Cl, Br, I, C1-C5 alkoxy, dialkylamino or trifluoromethanesulfonate; a and b are each an integer from 0 to 3 provided that a+b=3; Rxe2x80x2 is a C1-C6 straight, cyclic or branched alkyl group, and R is a functionalizing group.
In yet another aspect, the invention is a chromatographic column having improved lifetime, comprising
a) a column having a cylindrical interior for accepting a packing material, and
b) a packed chromatographic bed comprising porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry of the formula SiO2/(R2pR4qSiOt)n or SiO2/[R6(R2rSiOt)m]n wherein R2 and R4 are independently C1-C18 aliphatic or aromatic moieties, R6is a substituted or unsubstituted C1-C18 alkylene, alkenylene, alkynylene or arylene moiety bridging two or more silicon atoms, p and q are 0, 1, or 2, provided that p+q=1 or 2, and that when p+q=1, t=1.5, and when p+q=2, t=1; r is 0 or 1, provided that when r=0, t=1.5, and when r=1, t=1; m is an integer greater than or equal to 2, and n is a number from 0.03 to 1, said porous hybrid silica chromatographic matrix having a chromatographically-enhancing pore geometry and average pore diameters of about 100 to 300 xc3x85, and said porous particles of hybrid silica have been surface modified.
In yet another aspect, the invention is a chromatographic column having improved lifetime, comprising
a) a column having a cylindrical interior for accepting a packing material, and
b) a packed chromatographic bed comprising porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry that have been surface modified with a surface modifier having the formula Za(Rxe2x80x2)bSixe2x80x94R, where Z=Cl, Br, I, C1-C5 alkoxy, dialkylamino or trifluoromethanesulfonate; a and b are each an integer from 0 to 3 provided that a+b=3; Rxe2x80x2 is a C1-C6 straight, cyclic or branched alkyl group, and R is a functionalizing group.
Another aspect of the invention is a method of preparation of porous particles of hybrid silica having a chromatographically-enhancing pore geometry, comprising the steps of
a) prepolymerizing a mixture of one or more organoalkoxysilanes and a tetraalkoxysilane in the presence of an acid catalyst to produce a polyorganoalkoxysiloxane;
b) preparing an aqueous suspension of said polyorganoalkoxysiloxane, said suspension further comprising a surfactant or combination of surfactants, and gelling in the presence of a base catalyst so as to produce porous particles; and
c) modifying the pore structure of said porous particles by hydrothermal treatment,
thereby preparing porous particles of hybrid silica having a chromatographically-enhancing pore geometry.
In an additional aspect, the invention is a porous particle of hybrid silica having a chromatographically-enhancing pore geometry, produced by the process of
a) prepolymerizing a mixture of one or more organoalkoxysilanes and a tetraalkoxysilane in the presence of an acid catalyst to produce a polyorganoalkyloxysiloxane;
b) preparing an aqueous suspension of said polyorganoalkyloxysiloxane, said suspension further comprising a surfactant or a combination of surfactants, and gelling in the presence of an base catalyst so as to produce porous particles; and
c) modifying the pore structure of said porous particles by hydrothermal treatment,
thereby producing porous particles of hybrid silica having a chromatographically-enhancing pore geometry.
The present invention will be more fully illustrated by reference to the definitions set forth below.
The language xe2x80x9cchromatographically-enhancing pore geometryxe2x80x9d includes the geometry of the pore configuration of the presently-disclosed porous inorganic/organic hybrid particles, which has been found to enhance the chromatographic separation ability of the material, e.g., as distinguished from other chromatographic media in the art. For example, a geometry can be formed, selected or constructed, and various properties and/or factors can be used to determine whether the chromatographic separations ability of the material has been xe2x80x9cenhancedxe2x80x9d, e.g., as compared to a geometry known or conventionally used in the art. Examples of these factors include high separation efficiency, longer column life, and high mass transfer properties (as evidenced by, e.g., reduced band spreading and good peak shape.) These properties can be measured or observed using art-recognized techniques. For example, the chromatographically-enhancing pore geometry of the present porous inorganic/organic hybrid particles is distinguished from the prior art particles by the absence of xe2x80x9cink bottlexe2x80x9d or xe2x80x9cshell shapedxe2x80x9d pore geometry or morphology, both of which are undesirable because they, e.g., reduce mass transfer rates, leading to lower efficiencies.
Chromatographically-enhancing pore geometry is found in hybrid particles containing only a small population of micropores. A small population of micropores is achieved in hybrid particles when all pores of a diameter of about  less than 34 xc3x85 contribute less than about 1100 m2/g to the specific surface area of the particle. Hybrid materials with such a low micropore surface area give chromatographic enhancements including high separation efficiency and good mass transfer properties (as evidenced by, e.g., reduced band spreading and good peak shape). Micropore surface area is defined as the surface area in pores with diameters less than or equal to 34 xc3x85, determined by mulitpoint nitrogen sorption analysis from the adsorption leg of the isotherm using the BJH method.
xe2x80x9cHybridxe2x80x9d, i.e., as in xe2x80x9cporous inorganic/organic hybrid particlesxe2x80x9d includes inorganic-based structures wherein an organic functionality is integral to both the internal or xe2x80x9cskeletalxe2x80x9d inorganic structure as well as the hybrid material surface. The inorganic portion of the hybrid material may be, e.g., alumina, silica, titanium or zirconium oxides, or ceramic material; in a preferred embodiment, the inorganic portion of the hybrid material is silica. As noted before, exemplary hybrid materials are shown in U.S. Pat. No. 4,017,528, the text of which is incorporated herein by reference. In a preferred embodiment where the inorganic portion is silica, xe2x80x9chybrid silicaxe2x80x9d refers to a material having the formula SiO2/(R2pR4qSiOt)n or SiO2/[R6(R2rSiOt)m]n wherein R2 and R4 are independently C1-C18 aliphatic or aromatic moieties (which may additionally be substituted with alkyl, aryl, cyano, amino, hydroxyl, diol, nitro, ester, ion exchange or embedded polar functionalities), R6is a substituted or unsubstituted C1-C18 alkylene, alkenylene, alkynylene or arylene moiety bridging two or more silicon atoms, p and q are 0, 1 or 2, provided that p+q=1 or 2, and that when p+q=1, t=1.5, and when p+q =2, t=1; r is 0 or 1, provided that when r=0, t=1.5, and when r=1, t=1; m is an integer greater than or equal to 2, and n is a number from 0.03 to 1, more preferably, 0.1 to 1, and even more preferably 0.2 to 0.5. R2 may be additionally substituted with a functionalizing group R.
The term xe2x80x9cfunctionalizing groupxe2x80x9d includes organic functional groups which impart a certain chromatographic functionality to a chromatographic stationary phase, including, e.g., octadecyl (C18) or phenyl. Such functionalizing groups are present in, e.g., surface modifiers such as disclosed herein which are attached to the base material, e.g., via derivatization or coating and later crosslinking, imparting the chemical character of the surface modifier to the base material. In an embodiment, such surface modifiers have the formula Za(Rxe2x80x2)bSixe2x80x94R, where Z=Cl, Br, I, C1-C5 alkoxy, dialkylamino, e.g., dimethylamino, or trifluoromethanesulfonate; a and b are each an integer from 0 to 3 provided that a+b=3; Rxe2x80x2 is a C1-C6 straight, cyclic or branched alkyl group, and R is a functionalizing group. Rxe2x80x2 may be, e.g., methyl, ethyl, propyl, isopropyl, butyl, t-butyl, sec-butyl, pentyl, isopentyl, hexyl or cyclohexyl; preferably, Rxe2x80x2 is methyl.
The porous inorganic/organic hybrid particles possess both organic groups and silanol groups which may additionally be substituted or derivatized with a surface modifier. xe2x80x9cSurface modifiersxe2x80x9d include (typically) organic functional groups which impart a certain chromatographic functionality to a chromatographic stationary phase. Surface modifiers such as disclosed herein are attached to the base material, e.g., via derivatization or coating and later crosslinking, imparting the chemical character of the surface modifier to the base material. In one embodiment, the organic groups of the hybrid particle react to form an organic covalent bond with a surface modifier. The modifiers can form an organic covalent bond to the particle""s organic group via a number of mechanisms well known in organic and polymer chemistry including but not limited to nucleophilic, electrophilic, cycloaddition, free-radical, carbene, nitrene, and carbocation reactions. Organic covalent bonds are defined to involve the formation of a covalent bond between the common elements of organic chemistry including but not limited to hydrogen, boron, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and the halogens. In addition, carbon-silicon and carbon-oxygen-silicon bonds are defined as organic covalent bonds, whereas silicon-oxygen-silicon bonds that are not defined as organic covalent bonds.
In another embodiment, silanol groups are surface modified with compounds having the formula Za(Rxe2x80x2)bSixe2x80x94R, where Z=Cl, Br, I, C1-C5 alkoxy, dialkylamino, e.g., dimethylamino, or trifluoromethanesulfonate; a and b are each an integer from 0 to 3 provided that a+b=3; Rxe2x80x2 is a C1-C6 straight, cyclic or branched alkyl group, and R is a functionalizing group. Rxe2x80x2 may be, e.g., methyl, ethyl, propyl, isopropyl, butyl, t-butyl, sec-butyl, pentyl, isopentyl, hexyl or cyclohexyl; preferably, Rxe2x80x2 is methyl. In certain embodiments, the organic groups may be similarly functionalized.
The functionalizing group R may include alkyl, aryl, cyano, amino, diol, nitro, cation or anion exchange groups, or embedded polar functionalities. Examples of suitable R functionalizing groups include C1-C30 alkyl, including C1-C20, such as octyl (C8), octadecyl (C18), and triacontyl (C30); alkaryl, e.g., C1-C4-phenyl; cyanoalkyl groups, e.g., cyanopropyl; diol groups, e.g., propyldiol; amino groups, e.g., aminopropyl; and alkyl or aryl groups with embedded polar functionalities, e.g., carbamate functionalities such as disclosed in U.S. Pat. No. 5,374,755, the text of which is incorporated herein by reference. Such groups include those of the general formula 
wherein l, m, o, r, and s are 0 or 1, n is 0, 1, 2 or 3 p is 0, 1, 2, 3 or 4 and q is an integer from 0 to 19; R3 is selected from the group consisting of hydrogen, alkyl, cyano and phenyl; and Z, Rxe2x80x2, a and b are defined as above. Preferably, the carbamate functionality has the general structure indicated below: 
wherein R5 may be, e.g., cyanoalkyl, t-butyl, butyl, octyl, dodecyl, tetradecyl, octadecyl, or benzyl. Advantageously, R5 is octyl, , dodecyl, or octadecyl. In a preferred embodiment, the surface modifier may be an organotrihalosilane, such as octyltrichlorosilane or octadecyltrichlorosilane. In an additional preferred embodiment, the surface modifier may be a halopolyorganosilane, such as octyldimethylchlorosilane or octadecyldimethylchlorosilane.
In another embodiment, the hybrid particle""s organic groups and silanol groups are both surface modified or derivatized. In another embodiment, the particles are surface modified by coating with a polymer. In certain embodiments, surface modification by coating with a polymer is used in conjunction with silanol group modification, organic group modification, or both silanol and organic group modification.
The term xe2x80x9caliphatic groupxe2x80x9d includes organic compounds characterized by straight or branched chains, typically having between 1 and 22 carbon atoms. Aliphatic groups include alkyl groups, alkenyl groups and alkynyl groups. In complex structures, the chains can be branched or cross-linked. Alkyl groups include saturated hydrocarbons having one or more carbon atoms, including straight-chain alkyl groups and branched-chain alkyl groups. Such hydrocarbon moieties may be substituted on one or more carbons with, for example, a halogen, a hydroxyl, a thiol, an amino, an alkoxy, an alkylcarboxy, an alkylthio, or a nitro group. Unless the number of carbons is otherwise specified, xe2x80x9clower aliphaticxe2x80x9d as used herein means an aliphatic group, as defined above (e.g., lower alkyl, lower alkenyl, lower alkynyl), but having from one to six carbon atoms. Representative of such lower aliphatic groups, e.g., lower alkyl groups, are methyl, ethyl, n-propyl, isopropyl, 2-chloropropyl, n-butyl, sec-butyl, 2-aminobutyl, isobutyl, tert-butyl, 3-thiopentyl, and the like. As used herein, the term xe2x80x9cnitroxe2x80x9d meansxe2x80x94NO2; the term xe2x80x9chalogenxe2x80x9d designates xe2x80x94F, xe2x80x94Cl, xe2x80x94Br or xe2x80x94I; the term xe2x80x9cthiolxe2x80x9d means SH; and the term xe2x80x9chydroxylxe2x80x9d means xe2x80x94OH. Thus, the term xe2x80x9calkylaminoxe2x80x9d as used herein means an alkyl group, as defined above, having an amino group attached thereto. Suitable alkylamino groups include groups having 1 to about 12 carbon atoms, preferably from 1 to about 6 carbon atoms. The term xe2x80x9calkylthioxe2x80x9d refers to an alkyl group, as defined above, having a sulfhydryl group attached thereto. Suitable alkylthio groups include groups having 1 to about 12 carbon atoms, preferably from 1 to about 6 carbon atoms. The term xe2x80x9calkylcarboxylxe2x80x9d as used herein means an alkyl group, as defined above, having a carboxyl group attached thereto. The term xe2x80x9calkoxyxe2x80x9d as used herein means an alkyl group, as defined above, having an oxygen atom attached thereto. Representative alkoxy groups include groups having 1 to about 12 carbon atoms, preferably 1 to about 6 carbon atoms, e.g., methoxy, ethoxy, propoxy, tert-butoxy and the like. The terms xe2x80x9calkenylxe2x80x9d and xe2x80x9calkynylxe2x80x9d refer to unsaturated aliphatic groups analogous to alkyls, but which contain at least one double or triple bond respectively. Suitable alkenyl and alkynyl groups include groups having 2 to about 12 carbon atoms, preferably from 1 to about 6 carbon atoms.
The term xe2x80x9calicyclic groupxe2x80x9d includes closed ring structures of three or more carbon atoms. Alicyclic groups include cycloparaffins or naphthenes which are saturated cyclic hydrocarbons, cycloolefins which are unsaturated with two or more double bonds, and cycloacetylenes which have a triple bond. They do not include aromatic groups. Examples of cycloparaffins include cyclopropane, cyclohexane, and cyclopentane. Examples of cycloolefins include cyclopentadiene and cyclooctatetraene. Alicyclic groups also include fused ring structures and substituted alicyclic groups such as alkyl substituted alicyclic groups. In the instance of the alicyclics such substituents can further comprise a lower alkyl, a lower alkenyl, a lower alkoxy, a lower alkylthio, a lower alkylamino, a lower alkylcarboxyl, a nitro, a hydroxyl, xe2x80x94CF3, xe2x80x94CN, or the like.
The term xe2x80x9cheterocyclic groupxe2x80x9d includes closed ring structures in which one or more of the atoms in the ring is an element other than carbon, for example, nitrogen, sulfur, or oxygen. Heterocyclic groups can be saturated or unsaturated and heterocyclic groups such as pyrrole and furan can have aromatic character. They include fused ring structures such as quinoline and isoquinoline. Other examples of heterocyclic groups include pyridine and purine. Heterocyclic groups can also be substituted at one or more constituent atoms with, for example, a halogen, a lower alkyl, a lower alkenyl, a lower alkoxy, a lower alkylthio, a lower alkylamino, a lower alkylcarboxyl, a nitro, a hydroxyl, xe2x80x94CF3, xe2x80x94CN, or the like. Suitable heteroaromatic and heteroalicyclic groups generally will have 1 to 3 separate or fused rings with 3 to about 8 members per ring and one or more N, O or S atoms, e.g. coumarinyl, quinolinyl, pyridyl, pyrazinyl, pyrimidyl, furyl, pyrrolyl, thienyl, thiazolyl, oxazolyl, imidazolyl, indolyl, benzofuranyl, benzothiazolyl, tetrahydrofuranyl, tetrahydropyranyl, piperidinyl, morpholino and pyrrolidinyl.
The term xe2x80x9caromatic groupxe2x80x9d includes unsaturated cyclic hydrocarbons containing one or more rings. Aromatic groups include 5- and 6-membered single-ring groups which may include from zero to four heteroatoms, for example, benzene, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, triazole, pyrazole, pyridine, pyrazine, pyridazine and pyrimidine, and the like. The aromatic ring may be substituted at one or more ring positions with, for example, a halogen, a lower alkyl, a lower alkenyl, a lower alkoxy, a lower alkylthio, a lower alkylamino, a lower alkylcarboxyl, a nitro, a hydroxyl, xe2x80x94CF3, xe2x80x94CN, or the like.
The term xe2x80x9calkylxe2x80x9d includes saturated aliphatic groups, including straight-chain alkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl substituted cycloalkyl groups, and cycloalkyl substituted alkyl groups. In certain embodiments, a straight chain or branched chain alkyl has 30 or fewer carbon atoms in its backbone, e.g., C1-C30 for straight chain or C3-C30 for branched chain. In certain embodiments, a straight chain or branched chain alkyl has 20 or fewer carbon atoms in its backbone , e.g, C1-C20 for straight chain or C3-C20 for branched chain, and more preferably 18 or fewer. Likewise, preferred cycloalkyls have from 4-10 carbon atoms in their ring structure, and more preferably have 4-7 carbon atoms in the ring structure. The term xe2x80x9clower alkylxe2x80x9d refers to alkyl groups having from 1 to 6 carbons in the chain, and to cycloalkyls having from 3 to 6 carbons in the ring structure.
Moreover, the term xe2x80x9calkylxe2x80x9d (including xe2x80x9clower alkylxe2x80x9d) as used throughout the specification and claims includes both xe2x80x9cunsubstituted alkylsxe2x80x9d and xe2x80x9csubstituted alkylsxe2x80x9d, the latter of which refers to alkyl moieties having substituents replacing a hydrogen on one or more carbons of the hydrocarbon backbone. Such substituents can include, for example, halogen, hydroxyl, alkylcarbonyloxy, arylcarbonyloxy, alkoxycarbonyloxy, aryloxycarbonyloxy, carboxylate, alkylcarbonyl, alkoxycarbonyl, aminocarbonyl, alkylthiocarbonyl, alkoxyl, phosphate, phosphonato, phosphinato, cyano, amino (including alkyl amino, dialkylamino, arylamino, diarylamino, and alkylarylamino), acylamino (including alkylcarbonylamino, arylcarbonylamino, carbamoyl and ureido), amidino, imino, sulfhydryl, alkylthio, arylthio, thiocarboxylate, sulfate, sulfonato, sulfamoyl, sulfonamido, nitro, trifluoromethyl, cyano, azido, heterocyclyl, aralkyl, or an aromatic or heteroaromatic moiety. It will be understood by those skilled in the art that the moieties substituted on the hydrocarbon chain can themselves be substituted, if appropriate. Cycloalkyls can be further substituted, e.g., with the substituents described above. An xe2x80x9caralkylxe2x80x9d moiety is an alkyl substituted with an aryl, e.g., having 1 to 3 separate or fused rings and from 6 to about 18 carbon ring atoms, e.g., phenylmethyl (benzyl).
The term xe2x80x9carylxe2x80x9d includes 5- and 6-membered single-ring aromatic groups that may include from zero to four heteroatoms, for example, unsubstituted or substituted benzene, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, triazole, pyrazole, pyridine, pyrazine, pyridazine and pyrimidine, and the like. Aryl groups also include polycyclic fused aromatic groups such as naphthyl, quinolyl, indolyl, and the like. The aromatic ring can be substituted at one or more ring positions with such substituents, e.g., as described above for alkyl groups. Suitable aryl groups include unsubstituted and substituted phenyl groups. The term xe2x80x9caryloxyxe2x80x9d as used herein means an aryl group, as defined above, having an oxygen atom attached thereto. The term xe2x80x9caralkoxyxe2x80x9d as used herein means an aralkyl group, as defined above, having an oxygen atom attached thereto. Suitable aralkoxy groups have 1 to 3 separate or fused rings and from 6 to about 18 carbon ring atoms, e.g., O-benzyl.
The term xe2x80x9camino,xe2x80x9d as used herein, refers to an unsubstituted or substituted moiety of the formula xe2x80x94NRaRb, in which Ra and Rb are each independently hydrogen, alkyl, aryl, or heterocyclyl, or Ra and Rb, taken together with the nitrogen atom to which they are attached, form a cyclic moiety having from 3 to 8 atoms in the ring. Thus, the term xe2x80x9caminoxe2x80x9d includes cyclic amino moieties such as piperidinyl or pyrrolidinyl groups, unless otherwise stated. An xe2x80x9camino-substituted amino groupxe2x80x9d refers to an amino group in which at least one of Ra and Rb, is further substituted with an amino group.
The present porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry generally have a specific surface area, as measured by N2 sorption analysis, of about 50 to 800 m2/g, preferably about 75 to 600 m2/g, more preferably about 100 to 200 m2/g. The specific pore volume of the particles is generally about 0.25 to 1.5 cm3/g, preferably about 0.4 to 1.2 cm3/g, more preferably about 0.5 to 1.0 cm3/g. The porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry have an average pore diameter of generally about 50 to 500 xc3x85, preferably about 60 to 500 xc3x85, more preferably about 100 to 300 xc3x85. The micropore surface area is less than about 110 m2/g, preferably less than about 105 m2/g, more preferably less than about 80 m2/g, and still more preferably less than about 50 m2/g.
Porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry may be made as described below and in the specific instances illustrated in the Examples. Porous spherical particles of hybrid silica may, in a preferred embodiment, be prepared by a multi-step process. In the first step, one or more organoalkoxysilanes such as methyltriethoxysilane, and a tetraalkoxysilane such as tetraethoxysilane (TEOS) are prepolymerized to form a polyorganoalkoxysiloxane (POS), e.g., polyalkylalkoxysiloxane, by co-hydrolyzing a mixture of the two or more components in the presence of an acid catalyst. In the second step, the POS is suspended in an aqueous medium in the presence of a surfactant or a combination of surfactants and gelled into porous spherical particles of hybrid silica using a base catalyst. In the third step, the pore structure of the hybrid silica particles is modified by hydrothermal treatment, producing an intermediate hybrid silica product which may be used for particular purposes itself, or desirably may be further processed below. The above three steps of the process allow much better control of the particle sphericity, morphology, pore volume and pore sizes than those described in the prior art, and thus provide the chromatographically-enhancing pore geometry.
In one embodiment of the invention, the surface organic groups of the hybrid silica are derivatized or modified in a subsequent step via formation of an organic covalent bond between the particle""s organic group and the modifying reagent. Alternatively, the surface silanol groups of the hybrid silica are derivatized or modified into siloxane functional groups, such as by reacting with an organotrihalosilane, e.g., octadecyltrichlorosilane, or a halopolyorganosilane, e.g., octadecyldimethylchlorosilane. Alternatively, the surface organic and silanol groups of the hybrid silica are both derivatized or modified. The surface of the thus-prepared material is then covered by the organic groups, e.g., alkyl, embedded during the gelation and the organic groups added during the derivatization process or processes. The surface coverage by the overall organic groups is higher than in conventional silica-based packing materials, and therefore the surface concentration of the remaining silanol groups in the hybrid silica is smaller. The resulting material, used as a stationary phase for LC, shows excellent peak shape for basic analytes, and better stability to alkaline mobile phases than silica-based packing materials.
Where the prepolymerization step involves co-hydrolyzing a mixture of the two or more components in the presence of an acid catalyst, the content of the organoalkoxysilane, e.g., organotrialkoxysilane can be varied, e.g, from about 0.03 to about 1.0 mole per mole, or more preferably, about 0.2 to about 0.5 mole per mole, of the tetraalkoxysilane. The amount of the water used for the hydrolysis can be varied, e.g., from 1.0 to 1.35 mole per mole of the silane. The silane, water and the ethanol mixture, in the form of a homogeneous solution, is stirred and heated to reflux under a flow of argon. After it is refluxed for a time sufficient to prepolymerize to form polyorganoalkoxysiloxane (POS), e.g., polyalkylalkoxysiloxane, the solvent and the side product, mainly ethanol, is distilled off from the reaction mixture. Thereafter, the residue is heated at an elevated temperature, e.g., in the range of 120 to 140xc2x0 C. under an atmosphere of argon for a period of time, e.g., 1.5 to 16 h. The residue is further heated at this temperature, e.g, for 1 to 3 h under reduced pressure, e.g., 10xe2x88x922-10xe2x88x923 torr, to remove any volatile species.
In the second step, the POS is suspended into fine beads in a solution containing water and ethanol at 55xc2x0 C. by agitation. The volume percent of ethanol in the solution is varied from 10 to 20%. A non-ionic surfactant such as Triton X-100 or Triton X-45 is added into the suspension as the suspending agent. Alternatively a mixture of Triton X-45 and low levels of sodium dodecyl sulfate (SDS) or tris(hydroxymethyl)aminomethane lauryl sulfate (TDS) is added into the suspension as the suspending agent. The surfactants, e.g., alkylphenoxypolyethoxyethanol, are believed to be able to orient at the hydrophobic/hydrophilic interface between the POS beads and the aqueous phase to stabilize the POS beads. The surfactants are also believed to enhance the concentration of water and the base catalyst on the surface of the POS beads during the gelation step, through their hydrophilic groups, which induces the gelling of the POS beads from the surface towards the center. Use of surfactants to modulate the surface structure of the POS beads stabilizes the shape of the POS beads throughout the gelling process, and minimizes or suppresses formation of particles having an irregular shapes, e.g., xe2x80x9cshell shapedxe2x80x9d, and inhomogeneous morphology.
It is also possible to suspend a solution containing POS and toluene in the aqueous phase, instead of POS alone. The toluene, which is insoluble in the aqueous phase, remains in the POS beads during the gelation step and functions as a porogen. By controlling the relative amount of toluene in the POS/toluene solution, the pore volume of the final hybrid silica can be more precisely controlled. This allows the preparation of hybrid silica particles having large pore volume, e.g., 0.8-1.2 cm3/g.
The gelation step is initiated by adding the basic catalyst, e.g., ammonium hydroxide into the POS suspension agitated at 55xc2x0 C. Thereafter, the reaction mixture is agitated at the same temperature to drive the reaction to completion. Ammonium hydroxide is preferred because bases such as sodium hydroxide are a source of unwanted cations, and ammonium hydroxide is easier to remove in the washing step. The thus-prepared hybrid silica is filtered and washed with water and methanol free of ammonium ions, then dried.
In one embodiment, the pore structure of the as-prepared hybrid material is modified by hydrothermal treatment, which enlarges the openings of the pores as well as the pore diameters, as confirmed by nitrogen (N2) sorption analysis. The hydrothermal treatment is performed by preparing a slurry containing the as-prepared hybrid material and a solution of organic base in water, heating the slurry in an autoclave at an elevated temperature, e.g., 143 to 168xc2x0 C., for a period of 6 to 28 h. The pH of the slurry can be adjusted to be in the range of 8.0 to 10.7 using concentrated acetic acid. The concentration of the slurry is in the range of 1 g hybrid material per 5 to 10 ml of the base solution. The thus-treated hybrid material is filtered, and washed with water and acetone until the pH of the filtrate reaches 7, then dried at 100xc2x0 C. under reduced pressure for 16 h. The resultant hybrid materials show average pore diameters in the range of 100-300 xc3x85. The surface of the hydrothermally treated hybrid material may be modified in a similar fashion to that of the hybrid material that is not modified by hydrothermal treatment as described in the present invention.
The surface of the hydrothermally treated hybrid silica contains organic groups, which can be derivatized by reacting with a reagent that is reactive towards the particles"" organic group. For example, vinyl groups on the particle can be reacted with a variety of olefin reactive reagents such as bromine (Br2), hydrogen (H2), free radicals, propagating polymer radical centers, dienes, and the like. In another example, hydroxyl groups on the particle can be reacted with a variety of alcohol reactive reagents such as isocyanates, carboxylic acids, carboxylic acid chlorides, and reactive organosilanes as described below. Reactions of this type are well known in the literature, see, e.g., March, J. xe2x80x9cAdvanced Organic Chemistry,xe2x80x9d 3rd Edition, Wiley, New York, 1985; Odian, G. xe2x80x9cThe Principles of Polymerization,xe2x80x9d 2nd Edition, Wiley, New York, 1981; the texts of which are incorporated herein by reference.
In addition, the surface of the hydrothermally treated hybrid silica also contains silanol groups, which can be derivatized by reacting with a reactive organosilane. The surface derivatization of the hybrid silica is conducted according to standard methods, for example by reaction with octadecyltrichlorosilane or octadecyldimethylchlorosilane in an organic solvent under reflux conditions. An organic solvent such as toluene is typically used for this reaction. An organic base such as pyridine or imidazole is added to the reaction mixture to catalyze the reaction. The product of this reaction is then washed with water, toluene and acetone and dried at 80xc2x0 C. to 100xc2x0 C. under reduced pressure for 16 h. The resultant hybrid silica can be further reacted with a short-chain silane such as trimethylchlorosilane to endcap the remaining silanol groups, by using a similar procedure described above.
More generally, the surface of the hybrid silica particles may be surface modified with a surface modifier, e.g., Za(Rxe2x80x2)bSixe2x80x94R, where Z=Cl, Br, I, C1-C5 alkoxy, dialkylamino, e.g., dimethylamino, or trifluoromethanesulfonate; a and b are each an integer from 0 to 3 provided that a+b=3; Rxe2x80x2 is a C1-C6 straight, cyclic or branched alkyl group, and R is a functionalizing group, and by coating with a polymer. Rxe2x80x2 may be, e.g., methyl, ethyl, propyl, isopropyl, butyl, t-butyl, sec-butyl, pentyl, isopentyl, hexyl or cyclohexyl; preferably, Rxe2x80x2 is methyl.
The functionalizing group R may include alkyl, alkenyl, alkynyl, aryl, cyano, amino, diol, nitro, cation or anion exchange groups, or alkyl or aryl groups with embedded polar functionalities. Examples of suitable R functionalizing groups include C1-C30 alkyl, including C1-C20, such as octyl (C8), octadecyl (C18), and triacontyl (C 30); alkaryl, e.g., C1-C4-phenyl; cyanoalkyl groups, e.g., cyanopropyl; diol groups, e.g., propyldiol; amino groups, e.g., aminopropyl; and alkyl or aryl groups with embedded polar functionalities, e.g., carbamate functionalities such as disclosed in U.S. Pat. No. 5,374,755, the text of which is incorporated herein by reference and as detailed hereinabove. In a preferred embodiment, the surface modifier may be an organotrihalosilane, such as octyltrichlorosilane or octadecyltrichlorosilane. In an additional preferred embodiment, the surface modifier may be a halopolyorganosilane, such as octyldimethylchlorosilane or octadecyldimethylchlorosilane. Advantageously, R is octyl or octadecyl.
Polymer coatings are known in the literature and may be provided generally by polymerization or polycondensation of physisorbed monomers onto the surface without chemical bonding of the polymer layer to the support (type I), polymerization or polycondensation of physisorbed monomers onto the surface with chemical bonding of the polymer layer to the support (type II), immobilization of physisorbed prepolymers to the support (type III), and chemisorption of presynthesized polymers onto the surface of the support (type IV). see, e.g., Hanson et al., J. Chromat. A656 (1993) 369-380, the text of which is incorporated herein by reference. As noted above, coating the hybrid material with a polymer may be used in conjunction with various surface modifications described in the invention.
The porous inorganic/organic hybrid particles have a wide variety of end uses in the separation sciences, such as packing materials for chromatographic columns (wherein such columns may have improved stability to alkaline mobile phases and reduced peak tailing for basic analytes), thin layer chromatographic (TLC) plates, filtration membranes, microtiter plates, scavenger resins, solid phase organic synthesis supports, and the like having a stationary phase which includes porous inorganic/organic hybrid particles having a chromatographically-enhancing pore geometry. The stationary phase may be introduced by packing, coating, impregnation, etc., depending on the requirements of the particular device. In a particularly advantageous embodiment, the chromatographic device is a packed chromatographic column, such as commonly used in HPLC.